1. Field of the Invention
The present invention relates to an apparatus and a method for estimating a Noise and Interference (NI) power in a communication system.
2. Description of the Related Art
In next generation communication systems, research is underway to provide users with services of transmitting and receiving large-capacity data at high speed. Representative examples of the next-generation communication system are an Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard and a Long Term Evolution (LTE) standard, which is currently being promoted by the 3rd Generation Partnership Project (3GPP). The IEEE 802.16m standard and the LTE standard use a Zadoff-Chu (ZC) sequence as a reference signal sequence in a ranging channel. The ZC sequence xu(n) may be expressed as equation 1 below.
                                                        x              u                        ⁡                          (              n              )                                =                      ⅇ                                          -                j                            ⁢                                                π                  ⁢                                                                          ⁢                                      un                    ⁡                                          (                                              n                        +                        1                                            )                                                                                        N                  ZC                                                                    ,                                  ⁢                  0          ≤          n          ≤                                    N              ZC                        -            1                                              (        1        )            
In addition, a signal sequence xu,v(n) obtained when the ZC sequence xu(n) is cyclic-shifted by “v” may be expressed as equation 2 below.xu,v(n)=xu((n+Cv)mod NZC)  (2)
In equations 1 and 2, “u” denotes a root index, “n” denotes a code index, “NZC” denotes a length of a ZC sequence, “v” denotes a cyclic shift index, and “C” denotes a cyclic shift length.
The ZC sequence has the characteristic that, while the cross-correlation between codes having different cyclic shifts in an equal root is zero, the cross-correlation between codes of different roots is √{square root over (NZC)}.
FIG. 1 is a block diagram illustrating a configuration of a ranging channel detector using a ZC sequence in a communication system according to the related art.
Referring to FIG. 1, a ranging channel detector includes a received signal pre-processing sub-block 101, a ranging signal sequence processing sub-block 130, a peak detection sub-block 105, and a timing estimation sub-block 107.
It is impossible to divide the resources of a ranging channel into subchannels or Physical Resource Units (PRUs) through a Fast Fourier transform (FFT) with another traffic channel or control channel. Therefore, in order to divide the resources of the ranging channel, a separate processing procedure must be performed on a received signal of each antenna before the FFT is performed.
The received signal pre-processing sub-block 101 extracts a ranging signal sequence, which is mapped to the resources of the ranging channel, from a received signal, and outputs the extracted ranging signal sequence to the ranging signal sequence processing sub-block 130. The ranging signal sequence processing sub-block 130 receives and multiplies the extracted ranging signal sequence having a length of “N” by a ranging signal sequence allocated to a base station, and performs an Inverse Fast Fourier transform (IFFT) on a resultant signal. Next, the ranging signal sequence processing sub-block 130 squares each component of the IFFT-processed signal, removes a phase component, and then outputs a resultant signal to the peak detection sub-block 105. Here, the allocated ranging signal sequence is transferred to a terminal, having a ranging channel detector, in the form of the square root of a ZC sequence through a super frame header.
The peak detection sub-block 105 receives the output of the ranging signal sequence processing sub-block 130, which has been generated according to each reception antenna. Then, the peak detection sub-block 105 adds values, which are received from each reception antenna, to each other according to each equal ZC sequence, and segments the received and added values according to each root index. That is, the peak detection sub-block 105 receives an N-length output signal of the ranging signal sequence processing sub-block 130, segments the received signal in a unit length “L” (wherein L=N/N), performs a cyclic shift on the segmented signals, and distinctively estimates impulse responses according to each ranging code in an order in which the cyclic shift is performed. Thereafter, the peak detection sub-block 105 detects a peak value among the estimated impulse responses. When the detected peak value exceeds a specific threshold value “NI,” representing noise and interference, the peak detection sub-block 105 determines that a corresponding ranging code has been received, and outputs a code index of the corresponding ranging code to the timing estimation sub-block 107.
The timing estimation sub-block 107 receives the code index of the corresponding ranging code, and uses the received code index as a timing estimation value for synchronization between the base station and the terminal.
FIG. 2 is a block diagram illustrating a configuration of an NI power estimator for estimating a specific threshold value “NI” according to the related art.
Referring to FIG. 2, an NI power estimator includes a segmentation unit 201, first to nth average time operation units 203, 205, 207, and 209, an arrangement unit 211, an average operation unit 217, and a multiplication unit 219. The specific threshold value “NI” is used in the peak detection sub-block 105 of FIG. 1 and the NI power estimator may be either included in the peak detection sub block 105 or separately constructed, or may be constructed to share the same function.
The segmentation unit 201 segments an N-length signal, which has been output from the ranging signal sequence processing sub-block 130, based on a length of “L” (wherein L=N/N) corresponding to each ranging code, and then outputs the segmented signals to the first to nth average time operation units 203, 205, 207, and 209 corresponding to the respective ranging codes. Each of the first to nth average time operation units 203, 205, 207, and 209 calculates a time average of impulse responses with respect to the L-length signal input according to each ranging code, and outputs the calculated time average to the arrangement unit 211. The arrangement unit 211 arranges average values of impulse responses according to each ranging code in order of the highest value to the lowest value, and outputs ranging codes in order of average value of impulse responses. In this case, among the arranged average values of impulse responses, ranging codes corresponding to x number of higher average values 213 and ranging codes corresponding to “N-x” number of lower average values 215 are separately output in distinction from each other.
Since a ranging channel is a random access channel, ranging codes which may be simultaneously received through one ranging channel can be randomly selected by a terminal from a total of N ranging codes. However, since a contention probability between the ranging codes must be controlled to be equal to or less than a predefined level, the number of ranging codes which may be simultaneously received through one physical ranging channel is actually very small. Therefore, the number of ranging codes which can be simultaneously received through one ranging channel is limited to “x,” as described above.
The average operation unit 217 receives the “N-x” number of lower ranging codes 215, calculates an overall average value of average values of impulse responses of the received ranging codes, and outputs the calculated overall average value to the multiplication unit 219. The multiplication unit 219 multiplies the input overall average value by a specific constant “TH” 221, and estimates a specific threshold value “NI” 223 used in the peak detection sub-block 105.
However, when an overall average value of average values of impulse responses of the “N-x” number of lower ranging codes 215 is set as an NI power value, a false alarm increases according to an increase of a Signal-to-Noise Ratio (SNR), as shown in FIGS. 3 and 4, which makes it impossible to satisfy a target false alarm. Here, the probability of the “false alarm” represents the probability that it is falsely determined that a base station has received a ranging code which a terminal has not transmitted. The probability of the false alarm may be expressed as equation 3.
                              False          ⁢                                          ⁢          alarm                =                                            Number              ⁢                                                          ⁢              of              ⁢                                                          ⁢              total              ⁢                                                          ⁢              false                        -            detections                                                                                                    Number                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    total                    ⁢                                                                                  ⁢                    candidate                    ⁢                                                                                  ⁢                    ranging                    ⁢                                                                                  ⁢                    codes                                    -                                                                                                      Number                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  total                  ⁢                                                                          ⁢                  transmitted                  ⁢                                                                          ⁢                  ranging                  ⁢                                                                          ⁢                  codes                                                                                        (        3        )            
In equation 3, the probability of the false alarm may be obtained by dividing a total number of times of detection of a false alarm by a difference between a total number of ranging codes transmittable by a terminal and a total number of ranging codes actually transmitted by the terminal.
FIG. 3 is a graph illustrating a probability of a false alarm as a function of a SNR in a synchronous ranging channel in standard environments according to the related art.
Referring to FIG. 3, a probability of a false alarm is illustrated when the SNR has a range from −10 to 5 dB, wherein it can be understood that, as the SNR increases, the probability of the false alarm increases at a high rate.
As an example of the standard environments, a case where a pedestrian is moving at a speed of 3 km/h is assumed, which is an environment defined in the IEEE 802.16 system standard. In addition, the probability of a false alarm is illustrated in FIG. 3 as a function of the SNR when an overall average value of average values of impulse responses of the “N−x” number of lower ranging codes 215 is set as an NI power value.
FIG. 4 is a graph illustrating a probability of a false alarm as a function of an SNR in an asynchronous ranging channel in standard environments according to the related art.
Referring to FIG. 4, a probability of a false alarm is illustrated when the SNR has a range from −11 to 0 dB. In FIG. 4, it can be understood that, although the probability of the false alarm does not increase at a high rate as the SNR increases, differently from the case of the synchronous ranging channel, the probability of the false alarm gradually increases as the SNR increases, and exceeds 0.1% at an SNR of −4.0 dB.
The reason why the probability of the false alarm increases as the SNR increases is that an NI power estimated by the NI power estimator is too small. When the NI power is estimated, an average of impulse responses to ranging codes, which belong to the same root as a ranging code transmitted from a terminal, but have cyclic shifts different from that of the ranging code transmitted from the terminal, becomes a value in which a cross-correlation is added by noise having a Gaussian random distribution. In such a situation, when an NI power is estimated by calculating an overall average value of average values of impulse responses of the “N-x” number of lower ranging codes 215, as illustrated in FIG. 2, ranging codes having an average value exceeding the calculated overall average value, among the average values of the impulse responses of the “N−x” number of lower ranging codes 215, do not correspond to ranging codes transmitted from a terminal, but are recognized as ranging codes received by a base station, thereby increasing the generation of the false alarm.
Therefore, a need exists for an apparatus and a method for reducing the generation of the false alarm by estimating an NI power in a communication system.